1. Field of the Invention
The present invention relates to a method of manufacturing a catalyst particularly useful for removing nitrogen oxides exhausted from transportable internal-combustion engines such as those used in gasoline-fueled automobiles and diesel-powered automobiles; stationary internal-combustion engines such as those used in cogeneration plants; combustors such as those used in boilers; furnaces of plants; etc., by reducing the nitrogen oxides to harmless gases.
2. Background Art
Generally, the exhaust gas exhausted from transportable or stationary internal-combustion engines, combustors, or furnaces of plants contains a large amount of nitrogen oxides (NO.sub.X) such as NO or NO.sub.2. These nitrogen oxides (NO.sub.X) are considered to cause not only photochemical smog but also damage to the human respiratory system.
Conventionally, there has widely been known a method for decreasing the NO.sub.X content in exhaust gas, wherein NO.sub.X is removed through reduction, by use of a catalyst, with carbon monoxide or hydrocarbons contained in the exhaust gas.
Catalysts commonly used for such purposes include those prepared by incorporating metals, by ion-exchange, impregnation, etc., into a carrier formed of zeolite such as crystalline aluminosilicate.
In particular, even under high gas hourly space velocity (GHSV) conditions, a crystalline aluminosilicate catalyst carrying copper as a metal component can effectively remove nitrogen oxides (NO.sub.X) from the exhaust gas containing a large amount of oxygen, by use of hydrocarbon as a reducing agent. Therefore, this type of catalyst is considered to be a promising catalyst for purifying exhaust gases from transportable or stationary internal-combustion engines.
However, the copper-on-crystalline aluminosilicate catalyst has poor durability to heat and steam; i.e., the valence, oxidation state, and dispersion state of copper incorporated in the catalyst are subject to change when the catalyst undergoes treatment with heat and steam. Thus, there remains a drawback that constant purification performance over prolonged periods is not obtained at temperatures of 600.degree. C. or more or when the exhaust gas contains a large amount of moisture.
To overcome this drawback, in Japanese Patent Application Laid-Open (kokai) No. 6-134314 the present inventors disclosed, as a catalyst for purifying exhaust gas and having remarkably enhanced durability, a catalyst comprising a pentasil-type crystalline aluminosilicate catalyst carrier carrying thereon a copper component and a phosphorus component.
With an ion-exchange method, which is suited for homogeneous incorporation of the copper component and the phosphorus component, the pH ranges of mother solutions which are proper for incorporation of respective components in required amounts differ from each other. Consequently, the pH range where both components are simultaneously incorporated in proper amounts is too narrow to be satisfactorily maintained by industrial apparatuses.
Although respective components may sequentially be incorporated within pH ranges suited for the respective components, an increased number of incorporation steps is not industrially advantageous at all.